Hydraulic shock absorbers or suspension dampers are used on vehicles to dampen spring oscillations caused by uneven road surfaces or vehicle accelerations. Hydraulic dampers absorb energy released by the springs and use hydraulic principles to transfer the energy to oppose the force of the springs. Dampers employ a piston within a cylinder, where the damper extends and retracts (compresses) in response to vehicle movements. Fluid is forced through a series of orifices and relief and check valves in order to control the motion of the piston and achieve damping.
Unfortunately, however, such dampers are static in that the orifices and the relief and check valves employ fixed openings and operate with the same damping characteristics for every compression and extension. Dampers should optimally be able to assume different operating characteristics under varying operating conditions. To correct this shortcoming, vehicle suspension systems have incorporated selectable or adaptable control systems. In these systems, the vehicle operator is able to select between firm or soft suspension ride modes. A small direct current motor within the damper opens or closes a valve which operates in parallel with the standard orifices and relief valves within the damper. As a result, the vehicle operator may modify the vehicle ride mode to accommodate personal preference.
Selectable control is limited, however, because the parallel valving must be set in either a fully on or a fully off position. It would be desirable for the control system to be proportionally controlled, so that the characteristics of the damper could change in relation to the varying road conditions. It would also be desirable for the control system to incorporate other factors which may affect the optimal suspension ride mode, such as vehicle speed, load, or the present position of the damper. Additionally, selectable control systems require several seconds for the selected ride mode to take effect. Thus, even if the operator desired to modify the ride mode for an upcoming change in road conditions, the selectable system may not be able to respond in time.
It can therefore be seen that a need exists for an improved pressure control valve that will provide proportional pressure control and that can be operated by a control system which can incorporate multiple variable inputs.